MODELLING OF SNOWMELT COMPONENT OF RUNOFF OF BHAGIRATHI CATCHMENT AT TEHRI

Abstract

The snow that accumulates in the Himalayas is one of the principal sources of water in the Indo-Gangetic plains of north India. Reliable estimates of the volume of water contained in the snowpack and its release are required for the efficient management of the water resources. One of the general characteristics of the snow fed basins is the accumulation of snow in winter and its depletion during the spring and summer months. The magnitude of the snow storage and its melting rate are controlled by the climatic conditions over the basin. During the melt season, the basin acts like a reservoir draining out continuously through the melt stream. The melt water routes through different stages before reaching the outlet of the basin. The melt water first percolates through the existing snowpack very slowly and reaches the ground surface. On the ground surface it travels as overland flow and then joins the melt stream. The present study was carried out with the objective of computing the snowmelt contribution in the river Bhagirathi at Tehri dam. The quantification of snow melt runoff is essential to secure long term water availability for this dam and effective reservoir operation. For computation of snowmelt runoff SNOWMOD model based on Degree-day approach was used. The study catchment up to Tehri dam comprising of an area of approx. 7299 sq. km. , lies between longitude 78°9'15" E to 79°24'55" E and latitude 30°20'20" N to 31°27'30" N. The elevation of the study area varies from 617 m to 6917 m. The basin is situated in the Western Himalayas and has an extensive snow cover during winter. Remote sensing techniques have been used to estimate the aerial extent of the snow cover. IRS-1 C/ 1 D LIS S III data has been analyzed using the ERDAS IMAGINE 9.1 software. The whole catchment area was divided into ten elevation bands of 650m each. The aerial extent of the snow cover has been evaluated for each elevation zone. The snow cover depletion curves were derived and the daily snow cover area was calculated. The daily runoff for the snow covered area and the snow free area was computed separately by conceptualizing the catchment as two sets of linear reservoirs. However, model was calibrated using the observed stream flow for the period from 1998-99 to 2000-01, model computes all the three components of runoff i.e. snowmelt iii runoff, rainfall Induced runoff and base flow separately. Model performance is adjudged based on statistical measures of Nash-Sutcliffe Efficiency (NSE) and percentage of volume difference. Analysis reveals that during calibration period, NSE varies between 0.78 to 0.81 with an average value of 0.79 and the difference in volume of observed and computed stream flow varying from (—) 3.99% to 2.87%. Also during validation period (2001/02 to 2003/04) NSE varies from 0.79 to 0.81 with an average of 0.8 and the difference in the volumes of computed and observed stream flow varying from (—) 2.36 to 1.8, which is even better than the calibration period. From the research findings it can be concluded that snow and glacier runoff contribution in Bhagirathi River at Tehri dam is 39.6% on annual basis and 3.5%, 41.5%, 52.7% and 32.7% for winter (November to January), spring (February to April), summer (May to July) and autumn (August to October) seasons respectively on seasonal basis. iv